You allow the turbo to do less work for the same manifold pressure, and thus get more effeciency. It is especially helpful if the turbo is working at its limits.
For example suppose your turbo is most efficient at a pressure ratio of 2.7:1. The pressure ratio is the ratio of the outlet pressure divided by the inlet pressure.
Lets suppose on a given day absolute air pressure is 12.5 psi. If you run the turbo at its ideal effeciency, you get a max boost of 12.5 x 2.7 = 33.75 psi.
Now you add 1 psi ram boost to that same setup, your inlet pressure is now 13.5 psi, so your max boost becomes 13.5 x 2.7 = 36.45 psi.
You gained 2.7 psi manifold pressure by adding 1 psi of ram boost!
Larry